Seasonal pitch change:

[Leslie Bartlett]

Do people notice tiny discrepencies of lever pressure on tuning pins?  We
never can get all of them perfect. My suspicion is that some of the unison
problem is simply the fact we are never perfect tuners, and as the piano
"flexes", when it settles back those discrepencies "come to the
surface".......  There are so many factors at work in tuning 3 strings
together..... It's a miracle, to me, anything stays.
les bartlett


   _____  

From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Andrew and Rebeca Anderson
Sent: Wednesday, March 07, 2007 8:11 PM
To: Pianotech List
Subject: Re: Seasonal pitch change:


I've been suspicious of soundboard deflection being entirely responsible
too, for different reasons.  Thanks to Ric we know we need to look
elsewhere.  I've also noticed a pattern to seasonal pitch changes that
involves the unisons, especially the three string unisons.  It usually don't
go out entirely the same way.  Sometimes in opposite directions.  Think
about it.  There's not a whole lot of board and rib to push up or sag down,
but there is a lot of board in the horizontal positions.  I wonder how much
bridge movement back and forth we get with hydration and dehydration?  By
the way, the pattern is the same on uprights too.

Andrew Anderson

At 06:26 PM 3/7/2007, you wrote:


So has anyone measured plate expansion and contraction with temperature
changes? Wouldn't that be the most likely suspect given this newfound
innocence of the soundboard? 

Jason

On 3/7/07, RicB <HYPERLINK
"mailto:ricb at pianostemmer.no"ricb at pianostemmer.no> wrote:


Hi List



I see I sent the wrong post, a copy of one earlier sent. Please excuse.


What I meant to write was the following.




I've been running some numbers and thinking a bit about this traditional


idea that vertical deflection of the strings is the main cause of pitch


change and thought some of you might find this interesting.



Given the following string lengths, all with identical back lengths


(50mm), and lengths from front termination to tuning pins (200mm) (to 


make the example simple) and assuming a 1 mm string deflection as the


starting point for all strings (also for simplicity) and calculating for


a roughly 50 cent pitch rise we get:



A string length of :



1400 mm needs 6 mm additional deflection which results in about 22 lbs


of downbearing.


1000 mm needs 5 mm additional deflection -->19 lbs downbearing


800 mm needs 4,5 mm additional deflection --> 18lbs downbearing 


500 mm needs 3,5 mm additional deflection --> 15 lbs downbearing


250 mm needs 2,5 mm additional deflection --> 13 lbs downbearing.


100 mm needs 1,75 mm additional deflection --> 13 lbs downbearing


50 mm needs 1,2 mm additional deflection --> 13 lbs downbearing. 



As you can see neither the amount of deflection needed to exact the


actual 50 pitch rise to begin with, nor the resulting downbearing


figures are within reason... which leaves one no choice but to admit


that something else is primarily responsible for seasonal pitch change. 



Its also good to note that if we are starting with a 1 mm deflection to


begin with.. which btw yeilds reasonable enough string deflection


angles, then the absolute most downward pitch change possible is when 


the panel flattens out and bearing becomes 0.  In this case the same


string lengths yeild :



1400 mm length --> -1 cent


1000 mm length --> -1,4 cent


800 mm length --> -1,7 cent


500 mm length --> -2,4 cent 


250 mm length --> -4 cent


100 mm length --> -7 cent


50 mm length --> -10 cent



These examples are simply illustrative of the kinds of things that


actually has to happen if the soundboard rise and fall is to account for 


most of the pitch change.  I apply all the resulting change on the


speaking length itself and do not account for any friction.  This is a


best case scenario. In reality the string will disperse some of any


change in tension caused by a change in vertical deflection... lessening 


the frequency change and resulting downbearing... but necessitating even


more vertical change for any give change in pitch.



I can not help but conclude... looking closer at the consequences....


that vertical rise and fall of the soundboard simply doesnt have much to 


do with the seasonal pitch change at all.



Tension change, and hence pitch change can come from an altering of the


relative positions of end points to each other... i.e. hitch pin and


tuning pin.  Pitch change can also happen without tension change if the 


speaking length is somehow altered.  Seems to me that ruling out


vertical deflection... one has to look to these to general conditions


for the explaination.



Cheers


RicB














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